This invention relates to a mobile radio communication system of the cellular system wherein a base station and a mobile station are connected to each other, and more particularly to a mobile radio communication system wherein soft hand-off processing is performed with regard to a forward link transmission signal when a mobile station moves between cells.
In a cellular system of the direct spread CDMA (Code Division Multiplex Access) system, in order to maximize the communication capacity for reverse link transmission signals, transmission power control of mobile stations is usually performed to avoid a near-far problem. To this end, in a descending channel, a base station transmits transmission power control signals to mobile stations. On the other hand, in a cellular system of the direct spread CDMA system, each of a base station and a mobile station has a plurality of demodulators so that it may perform separation of multi-path components and soft hand-off processing.
The soft hand-off processing is performed because, as shown in FIG. 5, when a mobile station 103 moves from a cell 101 to another cell 102 and comes to a position in an overlapping portion 104 between the two cells 101 and 102, deterioration of the quality of a signal which the mobile station 103 receives from a base station 105a of the cell 101 occurs. Thus, as the soft hand-off processing, substantially same signals are transmitted from the base station 105a and another base station 105b of the cell 102 to the mobile station 103 so that the mobile station 103 may demodulate and compose the signals from the two base stations 105a and 105b to obtain a signal of an improved signal quality. Also with regard to a reverse link, a signal transmitted from the mobile station 103 is de modulated and composed by the base stations 105a and 105b to improve a signal quality.
In this instance, transmission power control signals from the base stations 105a and 105b are each generated independently by the base stations 105a and 105b, respectively. This is because, when a transmission power control signal is transmitted only from the base station 105a to the mobile station 103 provided the mobile station 103 performs such processing as described above to control the transmission power, so that the base station 105a can receive the signal of an optimum level and then performs transmission with the controlled transmission power, there is the possibility that the signal transmitted from the mobile station 103 may be an unexpected signal and may be a significant disturbance to the other base station 105b. Then, the mobile station 103 demodulates the transmission power control signals from the two base stations 105a and 105b independently of each other and composes the demodulated transmission power control signals. More particularly, the mobile station 103 increases its transmission power only when, for example, the transmission power control signals from both of the base stations 105a and 105b exhibit "up", but when the transmission power control signal from at least one of the base stations 105a or 105b exhibits "down", the mobile station 103 decreases the transmission power. The mobile station 103 transmits a reverse link signal with an appropriate transmission power to the base stations 105a and 105b by receiving and demodulating the transmission power signals from the base stations 105a and 105b independently of each other in this manner.
As described above, for a cellular system which adopts the direct spread CDMA system, transmission power control of a mobile station is an essentially required technique. Further, in the cellular system, in order to maximize the communication capacity of the entire system, interference must be suppressed to the minimum. To this end, in the cellular system, soft hand-off processing is performed. Upon such soft hand-off, the mobile station 103 must perform transmission power control by demodulating and composing different transmission power control signals from the two base stations 105a and 105b.
In this instance, if the threshold level of the communication quality with which soft hand-off is to be started is set at a lower level, then the number of mobile stations which must perform soft hand-off when a communication resource for a plurality of channels is used increases, and this gives rise to a problem that the communication capacity of the entire system decreases.
On the contrary if the threshold level of the communication quality with which hand-off is to be started is set at a higher level, then since the probability that the mobile station 103 may be present in a region in which hand-off is to be performed decreases, the frequency of soft hand-off can be decreased, and thus the communication capacity of the entire system can be increased. However, the possibility that a communication signal to the adjacent base station 105b may be disturbed increases, and this gives rise to a problem that not only the capacity of the entire system is decreased due to the interference, but also the stability of the entire system is deteriorated.
Further, when the mobile station 103 is positioned in the region in which hand-off is to be performed, if the communication capacity of the base station 105b of the destination of the hand-off has reached its limit value, then soft hand-off is not performed, but hard hand-off by which the frequency channel is changed is performed. In this instance, if a communication channel is transmitted from the mobile station 103 in a reverse link in a frequency channel which has been used before the hard hand-off is performed, this gives an unexpected significant disturbance to communication signals being handled by the base station 105b, presenting a problem that communication of other mobile stations is disturbed.